This invention relates to microporous rare earth silicate compositions having a three dimensional framework structure, which contains at least silicon and one rare earth element as framework elements. The composition is represented by the empirical formula:
An(M1xe2x88x92zMxe2x80x2z)wSi1xe2x88x92yGeyOx.
Microporous crystalline compositions have many industrial uses and thus are the subject of numerous research projects in both industry and academia. The first microporous compositions to be synthesized were the zeolites which are crystalline aluminosilicate compositions, and which are formed from corner sharing AlO2 and SiO2 tetrahedra. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal, without significantly displacing any atoms which make up the permanent zeolite crystal structure. Investigation of zeolites and their structures led to the synthesis of non-zeolitic molecular sieves which are defined as crystalline compositions which contain in their framework structure, elements other than aluminum and silicon, but which exhibit the ion-exchange and/or adsorption characteristics of zeolites. These include: 1) crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440; 2) silicon substituted aluminophosphates as disclosed in U.S. Pat. No. 4,440,871, 3) metal substituted aluminophosphates as disclosed in U.S. Pat. No. 4,853,197; and 4) metallo zinc-phosphate compositions disclosed in U.S. Pat. No. 5,302,362. Non-zeolitic molecular sieves also include crystalline metal sulfide molecular sieves as disclosed in U.S. Pat. No. 4,880,761.
There are also several reports dealing with microporous rare earth silicates. For example, J. Rocha, et al. in Chem. Commun., (1997) 2103, disclosed the preparation and characterization of a composition having the bulk chemical formula of: Na2KYSi8O19.5H2O. This material was identified as AV-1 and shown to have yttrium and silicon in the framework. There are also a number of articles by S. M. Haile and co-workers in which a number of neodymium and yttrium silicates with either sodium or potassium as the alkali metal are disclosed. These articles are 1) S. M. Haile, et al., xe2x80x9cCrystallography and Composition of Some New Potassium-Neodymium Silicatesxe2x80x9d, Transaction of the American Crystallographic Association, Vol. 27, (1991) 77; S. M. Haile et al., xe2x80x9cSynthesis Structure and Ionic Conductivity of K3NdSi6O15xe2x80x9d, Material Research Society Symposium Proceedings, Vol. 210 (1991) 645; S. M. Haile et al., xe2x80x9cConductivity and Crystallography of New Alkali Rare Earth Silicates Synthesized as Possible Fast-Ion Conductorsxe2x80x9d, Solid State Ionics, 53-56 (1992) 1292-1301; S. M. Haile et al., xe2x80x9cAnisotropy in the Ionic Conductivity of K3NdSi3O8(OH)2xe2x80x9d, Fast Ion Transports and Solids, (1993) 315-326; S. M. Haile, et al. xe2x80x9cHydrothermal Synthesis of New Alkali Silicates I. Potassium Neodymium Phasesxe2x80x9d, Journal of Crystal Growth, 131 (1993) 352-372; S. M. Haile, et al., xe2x80x9cHydrothermal Synthesis of New Alkali Silicates, II; Sodium Neodymium and Sodium Yttrium Phasesxe2x80x9d, Journal of Crystal Growth, 131 (1993) 373-386; S. M. Haile et al. xe2x80x9cStructure of Na3NdSi6O15.H2Oxe2x80x94A Layered Silicate With Paths for Possible Fast Ion Conductionxe2x80x9d, Acta Cryst. (1997), B53, 7-17. A. N. Christensen, et al. in Acta Chemica Scandinavica, (1997) 51: 37-43 disclosed the synthesis of rare earth disilicates, while R. D. Shannon et al., in Phys. Chem. Minerals, 5, (1980) 245-253 disclosed the synthesis of sodium rare earth silicates which include yttrium, gadolinium, samarium, and dysprosium. Finally, R. D. Shannon et al. in Inorganic Chemistry, 17 (4), 958-964 (1978), disclose the ionic conductivity of a Na5YSi4O12 silicate.
Applicants have now synthesized crystalline microporous rare earth silicates, which are different from those enumerated above. These compositions can be described by the empirical formula:
An(M1xe2x88x92zMxe2x80x2z)wSi1xe2x88x92yGeyOx.
where A is a cation selected from the group consisting of alkali metals, alkaline earth metals, hydronium ion and mixtures thereof, xe2x80x9cnxe2x80x9d is the mole fraction of A and varies from about 0.5 w to about 4 w, M is at least one element selected from the group of rare earth elements, except that when xe2x80x9czxe2x80x9d is zero, M is not neodymium or yttrium, xe2x80x9czxe2x80x9d is the mole fraction of Mxe2x80x2 and varies from 0 to about 0.99, Mxe2x80x2 is a metal having a valence of +2, +3, +4 or +5, xe2x80x9cwxe2x80x9d is the mole fraction of the sum of M and Mxe2x80x2 and varies from about 0.1 to about 0.5, xe2x80x9cyxe2x80x9d is the mole fraction of germanium and varies from 0 to about 0.99 and xe2x80x9cxxe2x80x9d has a value such that it satisfies the valence of the framework elements. As will be shown in detail, the members of the family of rare earth silicates synthesized and characterized by applicants either have crystal structures which are different from those previously disclosed or contain rare earth elements in the structure which have not been synthesized before.
As stated, this invention relates to a new family of crystalline microporous rare earth silicates and a method of preparing them. Accordingly, one embodiment of the invention is a crystalline microporous composition having a three dimensional framework structure of at least silicon tetrahedral oxide units and at least one M oxide unit and having an empirical formula on an anhydrous basis of:
An(M1xe2x88x92zMxe2x80x2z)wSi1xe2x88x92yGeyOx.
where A is a cation selected from the group consisting of alkali metals, alkaline earth metals, hydronium ion and mixtures thereof, xe2x80x9cnxe2x80x9d is the mole fraction of A and varies from about 0.5 w to about 4 w, M is at least one element selected from the group of rare earth elements, except when xe2x80x9czxe2x80x9d is zero, M is not neodymium or yttrium, xe2x80x9czxe2x80x9d is the mole fraction of Mxe2x80x2 and varies from 0 to about 0.99, Mxe2x80x2 is a metal having a valence of +2, +3, +4 or +5, xe2x80x9cwxe2x80x9d is the mole fraction of the sum of M and Mxe2x80x2 and varies from about 0.1 to about 0.5, xe2x80x9cyxe2x80x9d is the mole fraction of germanium and varies from 0 to about 0.99 and xe2x80x9cxxe2x80x9d has a value such that it satisfies the valence of the framework elements.
Another embodiment of the invention is a process for preparing the rare earth silicates described above. The process comprises forming a reaction mixture containing reactive sources of A, M, silicon, optionally Mxe2x80x2 and optionally germanium, at a temperature and a time sufficient to form the crystalline composition, the mixture having a composition expressed in terms of mole ratios of oxides of:
aA2/mO:1xe2x88x92bMOh/2:bMxe2x80x2Og/2:1xe2x88x92cSiO2:cGeO2:dH2O
where xe2x80x9caxe2x80x9d has a value of about 1 to about 50, xe2x80x9cmxe2x80x9d is the valence of A and has a value of +1 or +2, xe2x80x9cbxe2x80x9d has a value from 0 to less than 1.0, xe2x80x9cgxe2x80x9d has a value of +2, +3, +4 or +5, xe2x80x9chxe2x80x9d has a value of +3 or +4, xe2x80x9ccxe2x80x9d has a value from zero to less than 1.0 and xe2x80x9cdxe2x80x9d has a value from about 30 to about 2000.
These and other objects and embodiments will become more apparent after the following detailed description of the invention.
Applicants have prepared crystalline and microporous rare earth silicates by a hydrothermal synthesis at a relatively low temperature and a relative high pH. Several of these rare earth silicates are characterized by x-ray diffraction patterns which are not associated with any known structures. The rare earth silicates have a three-dimensional framework structure of at least silicon tetrahedral oxide units and at least one rare earth oxide unit. The rare earth metals have a valence of +3 or +4 and a coordination number of 6, 7 or 8. These compositions are described on an anhydrous basis by the empirical formula:
An(M1xe2x88x92zMxe2x80x2z)wSi1xe2x88x92yGeyOx.
In this formula, xe2x80x9cAxe2x80x9d, which acts as both a charge balancing cation and a structure directing cation is selected from the group consisting of alkali metals, alkali earth metals, hydronium ion, and mixture thereof. Preferred alkali metals are sodium, potassium, and mixtures thereof, while preferred alkaline earth metals are barium and strontium. The value of xe2x80x9cnxe2x80x9d which is the mole fraction of A varies from about 0.5 w to about 4 w and is chosen such that the valence neutrality of the composition is achieved. The framework structure is composed of silicon, optionally germanium, at least one rare earth element (M) and optionally an Mxe2x80x2 metal. The amount of germanium present is represented by xe2x80x9cyxe2x80x9d which has a value from zero to less than 1.0, which therefore means that the amount of silicon is equal to 1xe2x88x92y. Similarly, the amount of Mxe2x80x2 metal is represented by xe2x80x9czxe2x80x9d which has a value from zero to about 0.99 and thus the amount of rare earth element present is represented by 1xe2x88x92z. The total mole fraction of (M+Mxe2x80x2) is represented by xe2x80x9cwxe2x80x9d which has a value from about 0.1 to about 0.5. Finally, the amount of oxygen present is represented by xe2x80x9cxxe2x80x9d which has a value such that it satisfies the valence of the framework elements. The rare earth elements which are represented by M include yttrium, ytterbium, neodymium, praseodymium, samarium, gadolinium, terbium, dysprosium, holmium, europium, lutetium, promethium, erbium, cerium, lanthanum and thulium. It should be pointed out that when more than one rare earth element is present in the composition, it is the total of the metals which will equal (1xe2x88x92z). That is,
M1xe2x88x92z=M1a+M2b+M3c+ . . . where a+b+c+ . . . =1xe2x88x92z.
Similarly, more than one Mxe2x80x2 metal can be present and each Mxe2x80x2 metal can have different valences, with the total amount of the Mxe2x80x2 metals being equal to z. This is represented by the following equation:
Mxe2x80x2z=M1xe2x80x2axe2x80x2+M2xe2x80x2bxe2x80x2+M3xe2x80x2cxe2x80x2+ . . . where axe2x80x2+bxe2x80x2+cxe2x80x2+ . . . =z.
The Mxe2x80x2 metals which can be substituted into the framework include those having a valence of +2, +3, +4 or +5. Examples of these metals include without limitation, zinc (+2), iron (+3), scandium (+3), cobalt (+3), zirconium (+4), titanium (+4), niobium (+5), tantalum (+5) and antimony (+5).
The rare earth silicate microporous compositions described herein are prepared by a hydrothermal crystallization of a reaction mixture prepared by combining reactive sources of silicon, rare earth (M) element, optionally an Mxe2x80x2 metal, optionally germanium, at least one cation and water. Silicon sources include, without limitation, colloidal silica, fumed silica, tetraethylorthosilicate and sodium silicate. Sources of the M metals include, but are not limited to metal halides, metal nitrates, metal acetates, metal oxides, metal hydrous oxides and mixtures thereof. Specific examples of rare earth compounds include, without limitation, ytterbium chloride, ytterbium oxide, ytterbium nitrate, ytterbium sulfate octahydrate, ytterbium carbonate and ytterbium oxalate. Specific examples for the other rare earths include without limitation the chloride, oxide, nitrate, sulfate and carbonate compounds. Sources of the Mxe2x80x2 metals also include without limitation the halides, nitrates, acetates, oxides, hydrous oxide and mixtures thereof. Specific examples include without limitation zinc chloride, zinc acetate, iron (III) chloride, iron (III) acetate, cobalt (III) bromide, cobalt (III) acetate, scandium (III) chloride, zirconium oxychloride, zirconium oxyacetate solution, zirconium butoxide, titanium (IV) chloride, titanium (III) chloride solution, niobium (V) chloride, hydrous niobium oxide, tantalum (V) chloride, tantalum ethoxide, antimony (V) oxide, and antimony (V) chloride. Germanium sources include without limitation germanium oxide, germanium alkoxides and germanium tetrachloride. Alkali sources, include without limitation, potassium hydroxide, sodium hydroxide, rubidium hydroxide, cesium hydroxide, sodium carbonate, potassium carbonate, rubidium carbonate, cesium carbonate, sodium halide, potassium halide, rubidium halide, cesium halide, sodium acetate, potassium acetate, cesium acetate and rubidium acetate. Alkaline earth metal sources include without limitation: calcium hydroxide, barium hydroxide, calcium chloride, etc.
Generally, the hydrothermal process used to prepare the rare earth silicate microporous compositions of this invention involves forming a reaction mixture containing reactive sources of the desired components. The reaction mixture can be described in terms of molar ratios of the oxides by the formula:
xe2x80x83aA2/mO:1xe2x88x92bMOh/2:bMxe2x80x2Og/21xe2x88x92cSiO2:cGeO2:dH2O
where xe2x80x9caxe2x80x9d has a value of about 1 to about 50 and preferably from about 1 to about 20, xe2x80x9cmxe2x80x9d is the valence of A and has a value of +1 or +2, xe2x80x9cbxe2x80x9d has a value from zero to less than 1.0, xe2x80x9cgxe2x80x9d is the valence of Mxe2x80x2 and has a value of +2, +3, +4 or +5, xe2x80x9chxe2x80x9d is the valence of M and has a value of +3 or +4; xe2x80x9ccxe2x80x9d has a value from zero to less than 1.0 and xe2x80x9cdxe2x80x9d has a value from about 30 to about 2000. The basicity of the mixture is controlled by adding excess alkali hydroxide and/or basic compounds of the other constituents of the mixture. Having formed the reaction mixture, it is next reacted at a temperature of about 50xc2x0 C. to about 300xc2x0 C. and preferably from about 150xc2x0 C. to about 250xc2x0 C. for a period of about 1 hr. to about 30 days in a sealed reaction vessel under autogenous pressure. Optionally, the reaction mixture can be maintained at ambient temperature for a time of about 1 hr. to about 7 days before heating. After crystallization is complete, the solid product is isolated from the heterogeneous mixture by means such as filtration or centrifugation and then washed with deionized water or dilute alkali metal hydroxide solution and dried in air at ambient temperature up to about 100xc2x0 C. The crystalline rare earth silicates prepared by the process described above are characterized by a three-dimensional framework structure of SiO2 tetrahedral oxide units, at least one rare earth metal oxide unit, optionally an Mxe2x80x2 metal oxide unit and optionally GeO2 tetrahedral oxide units. Further, the rare earth metals are 6, 7 or 8 coordinate and the Mxe2x80x2 metals are 4, 5 or 6 coordinate. Finally, these microporous compositions are characterized in that they have crystallographically uniform pores generally having a diameter greater than about 2.6 xc3x85 and usually from about 2.6 to about 15 xc3x85.
As synthesized, the molecular sieves of this invention will contain some of the structure directing or charge-balancing cations in the pores. These metals are described as exchangeable cations meaning that they can be exchanged with other (secondary), i.e., different cations (herein referred to as Axe2x80x2). Generally, the A exchangeable cations can be exchanged with Axe2x80x2 cations, which include alkali metal cations (K+, Na+, Rb+, Cs+), alkaline earth cations (Mg2+, Ca2+, Sr2+, Ba2+), hydronium ion, ammonium ion, transition elements having a +2 or +3 valence, rare earth metals having a valence of +2 or +2 and mixtures thereof. The methods used to exchange one A cation with a different Axe2x80x2 cation are well known in the art and involve contacting the molecular sieve with a solution containing the desired Axe2x80x2 cation at exchange conditions. Exchange conditions include a temperature of about 25xc2x0 C. to about 100xc2x0 C. and a time of about 20 minutes to about 30 hours. More than one exchange may be necessary to achieve the desired level of Axe2x80x2.
The crystalline compositions of this invention are capable of separating mixtures of molecular species based on the molecular size (kinetic diameters) or on the degree of polarity of the molecular species. When the separation of molecular species is based on molecular size, the crystalline microporous composition is chosen in view of the dimensions of its pores such that at least the smallest molecular species of the mixture can enter the intracrystalline void space while at least the largest species is excluded. The kinetic diameters of various molecules such as oxygen, nitrogen, carbon dioxide, carbon monoxide are provided in D. W. Breck, Zeolite Molecular Sieves, John Wiley and Sons (1974), p. 636.
When the separation is based on degree of polarity, it is generally the case that the more hydrophilic crystalline composition of this invention will preferentially adsorb the more polar molecular species of a mixture having different degrees of polarity even though both molecular species can communicate with the pore system of the crystalline material. For example water, which is more polar, will be preferentially adsorbed over common hydrocarbon molecules such as paraffins, olefins, etc. Thus, the crystalline materials of this invention can be used as desiccants in such adsorption separation/purification processes as natural gas drying, cracked gas drying, etc.
To allow for ready reference, the different structure types and compositions of rare earth silicate microporous compositions have been given arbitrary designations of RESi-1 where the xe2x80x9c1xe2x80x9d represents a framework of structure type xe2x80x9c1xe2x80x9d. That is, one or more rare earth silicate microporous composition with different empirical formulas can have the same structure type.
The X-ray patterns presented in the following examples were obtained using standard X-ray powder diffraction techniques. The radiation source was a high-intensity X-ray tube operated at 45 kV and 35 ma. The diffraction pattern from the copper K-alpha radiation was obtained by appropriate computer based techniques. Flat compressed powder samples were continuously scanned at 2xc2x0 (2xcex8) per minute from 2xc2x0 to 70xc2x0 (2xcex8). Interplanar spacings (d) in Angstrom units were obtained from the position of the diffraction peaks expressed as 2xcex8 where xcex8 is the Bragg angle as observed from digitized data. Intensities were determined from the integrated area of diffraction peaks after subtracting background, xe2x80x9cIoxe2x80x9d being the intensity of the strongest line or peak, and xe2x80x9cIxe2x80x9d being the intensity of each of the other peaks.
As will be understood by those skilled in the art, the determination of the parameter 2xcex8 is subject to both human and mechanical error, which in combination can impose an uncertainty of about xc2x10.4 on each reported value of 2xcex8 and up to xc2x10.5 on reported values for nanocrystalline materials. This uncertainty is, of course, also manifested in the reported values of the d-spacings, which are calculated from the xcex8 values. This imprecision is general throughout the art and is not sufficient to preclude the differentiation of the present crystalline materials from each other and from the compositions of the prior art. In some of the X-ray patterns reported, the relative intensities of the d-spacings are indicated by the notations vs, s, m and w which represent very strong, strong, medium, and weak, respectively. In terms of 100 X I/Io, the above designations are defined as w=0-15; m=15-60; s=60-80 and vs=80-100.
In certain instances the purity of a synthesized product may be assessed with reference to its X-ray powder diffraction pattern. Thus, for example, if a sample is stated to be pure, it is intended only that the X-ray pattern of the sample is free of lines attributable to crystalline impurities, not that there are no amorphous materials present.